Circuit board prototyping is a factor in hardware development. When designing hardware, the developer can go through several iterations of circuit board designs before finalizing their circuit board and having it mass manufactured. Hardware developers may outsource their circuit board designs, while paying large fees and waiting on long lead times, or fabricating the boards in-house, that can utilize dangerous and corrosive chemicals.
Beyond prototyping, mainstream circuit board patterning can be largely a subtractive process. In order to produce conductive patterns, a subtractive process can employ a chemical etching process whereby conductive material is removed from a single side, or both sides of a copper-clad base material. Holes, known as vias, used for component leads or for an electrical connection spanning from one conductive layer to another, can be provided using a Computer Numerical Control (CNC) drilling process. In the case of a via, the patterned and drilled board passes through a chemical plating process to form the electrical connection between layers.
As circuit designs and miniaturization requirements become more complex, two sides of a copper clad base material may be insufficient for such complex circuit designs. In these circuit boards, created by the subtractive process outlined above, several circuit patterns can be stacked and compressed into a single board to create multilayer circuitry. Stacked circuit boards commonly have in excess of sixteen stacked conductive layers, each separated by the base material. Each conductive layer in the final circuit board can significantly increase the setup and production costs as well as increasing the time to produce the circuit board.
Although chemical etching is a commonly used subtractive process for circuit board fabrication, the overall process can be time consuming, costly, and dangerous to the fabricator as it employs chemicals that can be very toxic and corrosive.
Other subtractive methods have been developed in an attempt to solve the problem of slow and dangerous circuit board prototyping, early on in the hardware development cycle. Isolation routing, for example, does not use chemicals to create the circuit boards but instead uses a computer guided drill bit to remove unwanted copper from a copper-clad base material. With limited setup and fast production times, this subtractive milling process is targeted towards in-house prototyping. However, this process may not be ideal for at least two reasons. First, it is generally limited to rigid substrates and can only produce single or dual sided boards. Second, routing of the copper-clad base can be noisy and can produce dust particles that are a harmful to the lungs.
Additive processes for fabricating circuit boards have drawn attention in the field of rapid prototyping. Unlike subtractive processes, additive techniques can offer the advantage of being virtually unrestricted in layer count due to the fact that material is used as it is needed. Furthermore, since material is added and not removed, there is generally less waste than within subtractive processes. Presently, additive manufacturing techniques with respect to conductive patterning on a base material have been primarily focused around inkjet printing. Inkjet printing technologies involve ejecting micro-drops of a printing fluid from an array of micro-nozzles onto a base material. The micro-drops can only be ejected onto the base material in the pattern specified in a digital file. Advantages of inkjet printing include small feature size combined with rapid processing and minimal setup utilizing a straightforward digital input file.
Conductive inks formulated for inkjet applications can include metallic nano-particles suspended in a solvent solution. Once the ink is ejected onto a base material, the solvent is evaporated leaving behind conductive metallic traces. The impedance of the metallic traces can be improved by post-processing methods such as laser sintering, photonic curing, or thermal curing. By alternating conductive and insulating inks as the printing fluids, layered conductive patterns can be created that are separated by insulating patterns to create complex designs.
Inkjet printing technologies can achieve high resolution performance by tightly controlling the ink's rheology and ensuring that the ink is relatively free of contaminants. The micro-nozzles, from which the printing fluid is ejected can be prone to contaminant buildup. This can be particularly true for conductive fluids. As the conductive ink is exposed to air at the nozzle opening, the solvent evaporates and can leave behind metallic residue around the nozzle. This residue can cause the micro-drops to be ejected at sharp angles or can even clog the nozzle entirely.
To combat the effects of residue build up and to maintain proper printing performance, the metallic content of the printing fluid can be kept relatively low at about 10% to about 30% by weight. By lowering the metallic content of the fluid, the amount of metallic content deposited on the base material is also lowered. As a result, the printed traces can lack the sufficient metal content needed for good electrical performance. Consequently, the metal content of the printed traces can be substantially increased by building up the trace thickness. This can be accomplished by repeatedly depositing conductive fluid over the same area to build up trace thickness and hence improve conductivity, as conductivity is a function of cross sectional area. In addition to being afflicted by poor conductivity, the traces formed by inkjet printing can have low melting temperatures, therefore cannot withstand high temperatures employed by popular soldering techniques such as wave, soldering iron, or reflow to attach functional elements onto the circuit board pattern. Further complications with the inkjet method includes the fluidic nature of these conductive inks, which can create problems with the interfacial surface energy between the ink and substrate material. To elaborate, the polarity of the ink solvent can lead to either over-wetting or under-wetting of the substrate material, which in turn can create poorly resolved traces or peeling of the traces after curing.
Another additive circuit board manufacturing method is screen printing, a technique that uses a woven mesh stencil to transfer conductive paste onto the desired printing material. The stencil forms the desired printed circuit pattern such that conductive paste is transferred through the open areas of the stencil and onto the base material to create an identical circuit pattern. A blade or squeegee is scraped across the stencil, forcing conductive paste to be transferred through the woven-mesh openings and onto the imposed printing material. This technique can allow for fine printed features, and is limited by the resolution of the circuit pattern on the stencil.
However, creating a stencil with the desired circuit pattern for screen printing can be expensive and time consuming. Screen printing is typically based on a photolithographic process that first coats a woven mesh screen in a photosensitive emulsion, and allows the emulsion to solidify through a thermal curing process. Next, a transparency with the printed circuit pattern is secured onto the cured emulsion and exposed to UV light. The transparency acts as a mask, such that areas covered by the pattern on the transparency are protected, while the rest of the emulsion is exposed to the UV light. The protected emulsion can then be washed off by chemicals to leave only the negative of the desired circuit pattern in the emulsion. This can act as a mask for the conductive paste that is to be screen printed across the woven mesh. This process of creating a stencil can be used every time a new circuit pattern is designed for printing.
Screen printing can utilize conductive Polymer Thick Film (PTF) pastes for printing a circuit pattern. Unlike conductive inks used in inkjet printing, conductive PTF pastes use higher viscosities and are therefore not primarily made of solvents. Polymeric materials, such as epoxy resin, typically act as the base in which metallic particles are suspended. A conductive PTF paste is typically composed of about 40% to about 98% metallic nano or micro particles, and the remaining about 2% to about 60% is polymeric material and additives. The combination of high solid content and polymeric material usually gives these screen printing pastes much higher viscosities of, for example, about 20 KcP to about 200 KcP. These higher viscosities can allow the pastes to hold their shape once printed which in turn allows fine features to be printed on the desired material. In addition, traces created by conductive PTF pastes are typically much taller than the sub-micron traces printed by inkjet applications. This increased cross-sectional area, along with the high metallic content, can provide improved conductivity and solderability over conductive inks utilized in inkjet printing. Overall it can be understood that in prototyping applications, where new circuit patterns are frequently used, a technique such as screen printing generally lacks practicality due to multiple stencil iterations.
In sum, the prior art techniques for prototyping circuit boards have various deficiencies. Outsourcing production of the circuit board design can come with elevated costs and lead times, while in-house prototyping techniques can be tedious and dangerous. Additive fabrication techniques such as inkjet printing utilize solvent-based conductive inks that can be unreliable and have poor electrical and structural properties. Although conductive PTF pastes used in screen printing applications have shown reasonable conductivity and solderability, the screen printing technique is not generally suitable for iterative circuit board designs, due to an abundant need for stencils.